1. Field of the Invention
The present invention relates to a developing unit included in an image forming apparatus such as a copying machine, a facsimile, and a printer, and to an image forming apparatus and a process cartridge having the developing unit.
2. Description of the Related Art
Electrophotographic image forming apparatuses are being demanded to provide higher image quality, as well as to be smaller in size and to have a longer lifetime. As the lifetime of a developing unit is extended, a large amount of toner is to be consumed. However, if a space for storing therein a large amount of toner is reserved in the developing unit, the size of the developing unit will be increased. Furthermore, if the amount of the toner stored in the developing unit is increased, more frictional heat is generated in stirring the toner. Accumulated heat could cause the toner resin to melt and to clump together, and might result in deterioration of image quality. Moreover, although the toner has a granularity distribution spreading across a certain range, the toner granules with smaller diameters tend to be consumed earlier in the development. Thus, the toner granules with larger diameters, not consumed while the development is performed repeatedly, are left behind, and the large-diameter toner granules come to occupy a larger ratio in the toner stored in the developing unit. If the development is performed using the toner having a granularity distribution shifted toward larger diameters, image quality might be deteriorated. If more toner is stored in the developing unit, because the development is repeatedly performed over a long time period without supplying toner containing granules having smaller diameters, image formation tends to be continued in a defective manner, using the toner with the increased ratio of larger diameter granules.
For the reasons described above, it is preferable to reduce the amount of the toner stored in the developing unit as much as possible within a range allowing high quality development. To this end, in a mainstream structure, the space for storing therein toner is reduced in the developing unit, and a toner container containing new toner is mounted replaceably on the developing unit, so that the toner is supplied from the toner container when the level of the toner remaining in the development unit becomes low.
Widely known as a developing unit to which toner is supplied in the manner described above is a single component developing unit including a developing roller, a supplying roller that carries toner, supplies the toner to the developing roller and collects the toner not used in the development from the developing roller, a toner storage that stores therein the toner, a stirring member that stirs the toner in the toner storage, a remaining toner amount detecting unit that detects the amount of the toner remaining in the toner storage, and a toner supplying unit that supplies the toner from a toner container to the toner storage based on the amount of the remaining toner. Furthermore, in the single component developing unit, it is known to provide a layer of sponge that is a porous body to the surface of the supplying roller, so that a sufficient amount of toner is carried and supplied to the developing roller, and the toner not used in the development can be well collected from the developing roller.
In such a single component developing unit, due to the stress applied to the toner stored in the toner storage during the repeated development process, additives sink into toner base resin, and cause the toner to deteriorate, such as reduction in flowability and chargeability of the toner. Before the toner is supplied, a large amount of deteriorated toner lies near the supplying roller in the toner storage. When new toner is supplied to the toner storage and the supplied toner with higher chargeability is brought into contact with the deteriorated toner with lower chargeability, the toner with higher chargeability is shifted to a higher charged state and the toner with lower chargeability is shifted to a lower charged state due to the interactions between the two. In this manner, bi-polarization of the amounts of charge is accelerated. Therefore, reversely charged toner, charged to the polarity opposite to the normal polarity to which the toner should be charged, comes to occupy a larger ratio in the toner near the supplying roller. If such reversely charged toner is supplied to the developing roller by the supplying roller and conveyed to a developing section, the toner might be attached to a non-image area of a latent image carrier, and might result in scumming caused after the toner is supplied.
As a method for reducing scumming caused by an increase of the reversely charged toner after the toner is supplied, Japanese Patent Application Laid-open No. H11-237772 discloses a method for improving the stirred condition of supplied toner and deteriorated toner. In this technology, a stirring member is provided in the toner storage, so that the supplied toner and the deteriorated toner are stirred sufficiently. In this manner, the ratio of the reversely charged toner is reduced in the toner stored near the supplying roller, and the ratio of the reversely charged toner is reduced in the toner to be supplied to the developing roller.
However, not necessarily sufficient is the suppressing effect achieved simply by reducing the ratio of the reversely charged toner near the toner supplying roller using the stirring member included in the toner storage. In particular, when a supplying roller including a sponge layer is used, scumming tends to occur after the toner is supplied. Generally, the toner carried on the supplying roller is gradually replaced with the toner near the supplying roller along a toner flow generated by the rotation of the supplying roller. However, the supplying roller having a sponge layer has a stronger toner carrying capability than that of a supplying roller having a layer with an even surface, so that it is difficult to replace the toner carried by the supplying roller along the toner flow generated by the rotations of the supplying roller. Therefore, even if the ratio of the reversely charged toner is reduced in the toner near the supplying roller, it is difficult for the supplying roller to carry the toner and supply the toner to the developing roller. The supplying roller keeps carrying the deteriorated toner on the sponge layer; and the deteriorated toner carried by the supplying roller is brought into contact with the supplied toner, which causes the supplied toner to be charged to the reversed polarity. As a result, the toner with a high ratio of the reversely charged toner is supplied to the developing roller.
Japanese Patent Application Laid-open No. 2009-156951 discloses a developing unit including a toner removing member that removes toner from the sponge layer of the supplying roller. In the developing unit, the toner removing member having a blade-shaped end is disposed at a fixed position where the toner removing member is kept deeply inside the sponge layer. When the supplying roller is rotated during image formation, the toner removing member removes the toner carried on the surface of or in the sponge layer. In such a development unit, the toner carried in the sponge layer of the supplying roller is removed during image formation, and thereafter the sponge layer carries new toner.
However, because the developing unit disclosed in Japanese Patent Application Laid-open No. 2009-156951 has a structure that keeps the toner removing member digging into the sponge layer of the supplying roller, deterioration of the sponge layer is accelerated and the components become less durable, which is against the extended lifetime of the developing unit. Furthermore, because the blade-shaped toner removing member is disposed at a fixed position, the toner removed from the supplying roller might remain depending on the position where the removing member is disposed, whereby a flow of the entire toner in the toner storage is prevented and the stirring condition may be worsened.
The present invention is made in consideration of the above, and an object of the present invention is to provide a developing unit, an image forming apparatus, and a process cartridge that can reduce scumming caused by the toner after having been supplied, while suppressing deterioration of a toner supplying member.